# What makes carbon dioxide gas "sticky" in pipes?

There's some perceived wisdom where I work that when flowing $$\ce{CO2}$$ gas in pipes it acts somewhat "sticky", in the sense that it lingers around longer and takes more $$\ce{N2}$$ or air to purge out than, say, methane or $$\ce{N2}$$.

Unfortunately, no one here seems to know the physical or chemical basis for this observed result. I do have some quantitative data through various flow sensors and IR gas sensors that show it really does stay around longer than $$\ce{N2}$$ or $$\ce{CH4}$$, so I don't think it's due to bad observation.

As you can imagine, googling "why is $$\ce{CO2}$$ sticky" is not very helpful. If anyone can explain to me why this happens, I'd be very grateful, or even just point me in the right direction to do my own research.

### EDIT:

Thanks for your responses so far.

Here's some more info on our gas system: we buy our $$\ce{CO2}$$ in large bottles at ~400 psi. We then feed it from outside through a pressure regulator that regulates it down to 2 bar (~29 psi) absolute pressure. All the pipes until this point are stainless steel.

It's then fed through a proportional valve and the pressure is held at 1030 mBar over a set of flow and gas sensors, then exhausted to atmosphere. The pipes in this section are a mix of Polyurethane tubes, and two proprietary rubber-like materials called Tygon and Viton tubing. These are held together with nylon fittings. All of this is at ambient temperature. In these pipes, only $$\ce{N2}$$, $$\ce{CO2}$$, and $$\ce{CH4}$$ flow.

Here are some graphs I collected of the system:

In this graph, the green & brown lines are each gas being flowed in the system. The two blueish lines are a rough analog of how much $$\ce{CO2}$$ is detected in the system. As you can see, even after 5 mins of methane and N2 purging, and continuous intermittent $$\ce{N2}$$ purges, $$\ce{CO2}$$ still creeps back into the system when no gas is flowing.

Here's a Graph of Gas Flow

The [Flow In] sensor is just after the 1030 Prop valve, and the [Flow Out] sensor is just before the exhaust. They are both identical Wheatstone bridge thermal flow sensors.

As you can see, there's a marked increase in flow loss (Flow In/Flow Out) from 2% to about 7% when CO2 is flowing, and some very strange transient spikes when I switch gasses to/from $$\ce{CO2}$$.

That's most of the data I've collected. Anecdotally, I've also noticed that the $$\ce{CO2}$$ valve is significantly harder to turn than the $$\ce{N2}$$ or $$\ce{CH4}$$ valves.

I hope this helps with the problem, and thank you again for your help so far.

• There are some cross-country CO2 pipelines ( like 18" diameter). They pose unusual fracture concerns for the steel pipe. While following the technology , I never encountered "sticky" considerations. Operating pressures are above the critical point. Feb 16 at 23:13
• @Andrew I have added some more supporting information. Let me know what you think. Feb 17 at 10:12

$$\ce{CO2}$$ solubility in water is about 100 times higher than other gasses. So if there is any moisture on the walls, $$\ce{CO2}$$ is much more likely to stay there.
$$\ce{CO2}$$ is also much more chemically active, forming lots of compounds through its carbonic acid. Some of these compounds are reversible, can absorb and emit $$\ce{CO2}$$ as its partial pressure or temperature changes.